1. Field of the Invention
The present invention relates to a scan line drive technology for a display panel in which light-emitting elements emitting light by means of current drive performed by organic light-emitting diodes (OLEDs), organic electroluminescences (OELs), and the like that are arranged in a matrix.
2. Background Information
A display unit in which OLED elements functioning as self luminous elements are arranged in a matrix has been known. The display unit in which OLED elements are used is a low power consumption unit, and requires no lighting component such as a backlight. In addition, the response speed of such a display unit is very fast. Therefore, it shows much promise as a future display unit.
A conventional driver for a display unit in which OLED elements are used will be hereinafter explained with reference to FIGS. 1 and 2. Note that the conventional driver shown in FIG. 2 is disclosed as prior art in Japan Patent Application Publication JP-A-2004-302025.
FIG. 1 is a diagram showing an equivalent circuit of an OLED element. FIG. 2 is a diagram showing a configuration of a display unit in which OLED elements are used and conventional drivers are included.
As shown in FIG. 1, an OLED element is expressed by an equivalent circuit comprised of a diode component E and a parasitic capacitance component Cp that is connected to the diode component E in parallel. In other words, the OLED element is a capacitive light emitting element.
In a display unit 40 shown in FIG. 2, m×n number of OELD elements E_11 to E_mn arranged in a matrix are coupled to intersections between m number of cathode lines (i.e., row lines) and n number of anode lines (i.e., column lines).
A driver (cathode driver 21) in the cathode side of the OLED elements includes m number of switching elements SW_10 to SW_m0 that are connected to m number of cathode lines H_1 to H_m, respectively. Each of the switching elements SW_10 to SW_m0 operates in response to a control signal output from a light-emitting control circuit (CONT) 11, and connects each of the cathode lines H_1 to H_m to a power supply potential VDD (high-level or simply H-level) or a ground potential (low-level or simply H-level). Reverse bias voltage is set to be applied to an OLED element connected to the cathode line that is connected to the H-level.
A driver (anode driver 30) in the anode side of the OLED elements includes n number of switching elements SW_01 to SW_0n that are connected to n number of anode lines V_1 to V_n, respectively. Each of the switching elements SW_01 to SW_0n operates in response to a control signal from the light-emitting control circuit 11, and connects each of the anode lines V_1 to V_n to n number of constant current sources CS_1 to CS_n, respectively, or the L-level.
For example, an OLED element E_21 emits light if a constant current source CS_1 is connected to a switching element SW_01 while a cathode line H_2 is scanned. If this is performed, forward bias is applied to a diode component of the OLED element E_21. Accordingly, the OLED element E_21 emits light.
In the conventional driver shown in FIG. 2, reset control is performed when each of the columns in the cathode side of the OLED elements arranged in a matrix is sequentially scanned. In other words, a reset period is set to be inserted in the period during which cathode lines are sequentially scanned in the reset control. In this reset period, all the cathode lines and anode lines are once set to be a reset potential (ground potential in FIG. 2).
FIG. 3 is a timing chart showing an operation of a conventional driver by which reset control is performed. FIG. 3 is comprised of two sub-charts (a) and (b). The sub-chart (a) shows signal waveforms of anode lines, and the sub-chart (b) shows signal waveforms of cathode lines.
As shown in the sub-chart (b) of FIG. 3, in the reset control, reset periods RS are set to be inserted between a period T1 during which a cathode line H_1 is scanned and a period T2 during which a cathode line H_2 is scanned, and between the period T2 and a period T3 during which a cathode line H_3 is scanned. For example, the cathode line H_1 is connected to the L-level during the period T1, and all the OLED elements connected to the cathode line H_1 emit light in response to current from the constant current sources CS_1 to CS_n. All the cathode lines excluding the cathode line H_1 are in the H-level during the period T1. Therefore, parasitic capacitance components in the OLED elements connected to the cathode lines H_2 and H_3, for instance, are in a charged state while the side thereof connected to the cathode line functions as a positive electrode. Based on this, electric charges built up in the parasitic capacitance components are discharged by once setting all the cathode lines and anode lines to be the ground voltage during the reset period RS immediately after the period T1. Because of this discharge, current instantaneously flows into the parasitic capacitance components of the OLED elements that should emit light from the cathode lines H_1, H_3, . . . H_m excluding the cathode line H_2 during the period T2. Thus, the parasitic capacitance components of the OLED elements that should emit light are charged.
However, false emission and/or destruction of the OLED element(s) may be caused in the conventional driver by which reset control is performed. The following is an explanation thereof.
As shown in FIG. 3, all the cathode lines in the conventional driver are in the L-level during the reset periods RS. At time t1, time t2, and time t3, when scanning of any of the cathode lines starts, the potentials of all the cathode lines excluding the target to be scanned are set to be changed from the L-level to the H-level.
For example, at the time t1, potential of the cathode line H_1 that is the target to be scanned during the period T1 is set to be the L-level, and potentials of the cathode lines H_2 and H_3 that are not the targets to be scanned during the period T1 are set to be changed from the L-level to the H-level. At this time, potentials of the cathode lines H_2 and H_3 extremely change. Therefore, parasitic capacitance components of the OLED elements connected to the cathode lines H_2 and H_3 instantaneously switch on. This results from the fact that impedance of the parasitic capacitance component is transiently reduced when extreme potential change occurs.
If the parasitic capacitance components of the OLED elements connected to the cathode lines H_2 and H_3 instantaneously switch on, the potentials of anode lines that should not originally be high will leap through the parasitic capacitance components (see the time t1 shown in the sub-chart (a) of FIG. 3). Accordingly, the OLED element coupled to the anode lines will falsely emit light. In addition, unintended high voltage will be applied to the anode lines due to the instantaneous switch-on of the parasitic capacitance components. Therefore, there is a possibility that the OLED elements will be destroyed. As shown in FIG. 3, the parasitic capacitance components of the OLED elements instantaneously switch on not only at the time t1, but also at the starting times of scanning the cathode lines excluding the time T1 (i.e., t2, t3 . . . ).
A display unit with approximately 4000 displayable colors was developed several years ago, and display of the colors is realized by 4-bit (i.e., 16 gradations) emission of each of the RGB light-emitting elements. Recently, the number of displayable colors has been remarkably increasing in display units in which OLED elements are used. For example, a display unit that can display 65000 colors or 260000 colors has been developed. In other words, a display unit in which each of the RGB light-emitting elements emits 5-bit (i.e., 32 gradations) or greater has been developed. The color gradation is determined by the pulse width modulation (PWM) period of current that follows through OLED elements. When the above described false emission occurs, false colors are generated by gradation changes in accordance with this false emission. In particular, as the number of bits of each of the RGB light-emitting elements in a display unit increases from five to six, the impact of the false emission on the false colors has been measurable.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved driver for a display panel in which a plurality of light-emitting elements are arranged in a matrix, the driver preventing false emission and/or destruction of the light-emitting elements from being caused when the above described reset control is performed during the scanning of row lines included in the display panel. It is also apparent to those skilled in the art from this disclosure that there exists a need for a method of manufacturing this improved driver. This invention addresses these needs in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.